Energy is the most important scientific and technological challenge facing humanity in the 21st century. In the long term, solar green energy is the only source of renewable and inexhaustible energy that has the capacity to fill humanity's technological needs. The present energy and environmental crises force people to think of innovative ways to solve problems by reducing energy consumption or increasing the use of renewable or clean energy sources. One such innovation would be the technology related to smart windows.
Conventional glass windows allow sunlight to filter through, naturally heating a room, and as a result can require an increase in the use of air conditioning, etc.
Compared with conventional glass windows, currently there are about five types of so called smart windows, i.e. liquid crystal windows, electrochromic windows, suspended particle based windows, thermochromic windows and photochromic windows.
The liquid crystal windows include polymer dispersed liquid crystal (PDLC) windows and polymer stabilized cholesteric texture (PSCT) windows, which are mainly used for privacy control. PDLC windows, which were invented at Kent State University in 1983, usually involve phase separation of nematic liquid crystal from a homogeneous liquid crystal containing an amount of polymer, sandwiched between two glasses coated with a transparent conductive material. When the electric filed is off, the liquid crystals in the window are randomly scattered. Light entering the windows does not have a clear path out. When a certain voltage is applied between the two conductive coatings, the liquid crystals homeotropically align and the window is transparent and the light can go through the window. However, PSCT windows usually include two kinds of PSCT, i.e. normal mode PSCT and reverse mode PSCT. In a normal PSCT window, on a focal conic state, light is scattered. If an electric field is applied to the liquid crystal, it turns to a homeotropic state, i.e. the liquid crystals reorient themselves parallel to each other along with the electric field and the window appears transparent, allowing light to pass through the device without light scattering. In reverse mode PSCT, the liquid crystal panel is transparent at zero electric field, whereas it is scattering (opaque) upon application of an electric field.
An electrochromic window generally comprises a metal ion conductor thin film sandwiched between two pieces of glass coated with different transparent electronic conductors. When a voltage is applied, moving ions from the counter-electrode to the electrochromic layer cause the color change. Reversing the voltage moves ions from the electrochromic layer back to the counter-electrode layer, restoring the window to its previous clear state. The glass may be programmed to absorb only part of the incident light, such as solar infrared.
Suspended particle based windows comprise microscropic molecular particles suspended in a liquid solution sandwiched between two pieces of glass coated with conducting material. When a voltage is applied the color of the window is changed to transparent, letting sunlight in. When the voltage is ‘off’, the particles return to rest randomly and the glass becomes tinted which is similar to the electrochromic windows.
Thermochromic windows such as those including “cloud gel” can change from a clear state to a diffused state upon heat, while reducing the transmission of solar heat. Such windows are thought to be able to reduce air conditioning costs when it's hot outside. However, a person cannot see through the window once it loses its transparency.
Photochromic windows respond to changes upon exposure to light, much like sunglasses that darken when you move from a dim light to a bright one. They work well to reduce glare from the sun, however they cannot control heat gain. For example, a photochromic window would darken more in the winter than in the summer, although solar heat would be beneficial in the winter.
Due to some problems with current window technologies, novel smart window technology with satisfactory functionality holds great promise in multi-functionality, reducing energy consumption and cutting air conditioning and heating loads in the future. Development of novel smart windows is driven by energy-efficiency demands. The Environmental Protection Agency has reported that an average household spends about 40 percent of its annual energy budget on heating and cooling costs. Office buildings now account for about one-third of all the energy used in the U.S., a quarter of which is lost through the inefficiency of standard windows to retain heat in the winter or deflect heat in the summer.
Liquid Crystals (LCs) represent a fascinating state of matter which combines order and mobility on a molecular and supermolecular level. The unique combination of order and mobility results in that LCs are typically “soft” and respond easily to external stimuli. The responsive nature and diversity of LCs provide tremendous opportunities as well as challenges for insight in fundamental science, and opens the door to various applications. The electro-optic response of rod-like or disk-like LCs, on which LC display industry is based, is just one example.
A chiral nematic LC, i.e. cholesteric LC, is a type of chiral LC with a characteristic helical structure. The helical superstructure can selectively reflect light according to Bragg's law. The wavelength λ of the selective reflection is defined by λ=np, where p is the pitch length of the helical structure and n is the average index of refraction of the LC material. The ability of a chiral dopant to twist an achiral nematic LC phase, i.e. helical twisting power (HTP, β), is expressed in the equation: β=(pc)−1 where c is the chiral dopant concentration.
A chiral nematic LC has three textures (states). A planar texture, where the director of the helical axis is perpendicular to the cell surface, can selectively reflect light, i.e. its optical state is reflective. If a low voltage pulse is applied normal to the cell surface, the focal conic texture is formed, where the director of the helical axis is more or less parallel to the cell surface. A random distribution of helical axes is characteristic of the focal conic texture which scatters the incident light in all directions. In this case, i.e. opaque state, you cannot see through the window. Furthermore, the focal conic state can block most ultraviolet rays of sunlight, which can eliminate the fading of furniture, carpets, drapes, artwork and other valuables indoors. If the electric field increased above a threshold value is applied, the focal conic texture is switched into a homeotropic texture where the helical structure is unwound with the liquid crystal director aligned in the cell normal direction. The homeotropic texture can let the incident light go through, i.e. one can see through the window.
Polymer networks can be formed by ultraviolet light induced polymerization of monomer(s). Polymer networks are formed from some quantity of reactive monomer(s) and photoinitiator in the cholesteric liquid crystal. The amount of chiral dopant can be adjusted to produce the desired cholesteric pitch. After the desired texture is established through the combination of surface preparations and applied field, the ultraviolet light is used to photopolymerize the sample. The morphology of the resulting polymer network mimics the textures of initial cholesteric mesophases in the preparation. With the presence of polymer networks, the liquid crystal material is broken up into small domains referred to as polydomains in the focal conic texture. The network influences the structure of the focal conic state and stabilizes initial states. Factors controlling morphology are related to LC texture, monomer concentration, photopolymerization temperature, UV intensity, exposure time etc. The presence of a polymer network provides similar advantages in enhancing the stability of the structure, aiding in the return of the LC orientation to the desired stable configuration, reducing the switching time, and helping to determine and maintain the poly-domain size.
Photochromism is usually used to describe compounds that undergo a reversible photochemical transformation where an absorption band in the visible electromagnetic spectrum changes dramatically in intensity or wavelength. In many cases, this can be described as a reversible change of color upon electromagnetic radiation. Photochromic compounds may be thermally reversible or thermally irreversible. Thermally reversible photochromic compounds such as spiropyrans, spirooxazines, benzopyrans, naphthopyrans are generally capable of switching from a first state, e.g. a colorless state, to a second state, e.g. a color state, under electromagnetic radiation and reverting back to the colorless state in response to thermal energy or another wavelength electromagnetic radiation, i.e. relaxing back to the colorless state. Thermally irreversible photochromic compounds such as diarylethenes and fulgides are generally capable of switching from a first state, e.g. a colorless state, to a second state, e.g. a color state, under an electromagnetic radiation and reverting back to the colorless state only under another electromagnetic radiation.